There are three good reasons this number was used. The first is the fact that in the Scriptures, the number of Man is 6 because Man was created on the sixth day. In contrast, the number of God is 7 because it represents completion and perfection, since on the seventh day of creation, God rested from His work which He declared to be good or perfect.

Now, in the Hebrew language, superlatives are expressed by repeating the word or phrase, the ultimate being its repetition three times. Thus, when Isaiah saw the Lord lifted up and exalted in the Temple, he heard Seraphim singing, "Holy, holy, holy is the Lord of hosts" (Isaiah 6:1-3). The number 666 therefore represents the number of Man carried to its zenith.

The use of this number is most appropriate because during the Tribulation Satan will work to exalt his man, the Antichrist, as the world's Messiah. Further, on the earth at that time there will be a demonic trinity who will represent the Humanistic aspirations of Man - Satan masquerading as God, the Antichrist pretending to be the Messiah, and the False Prophet mimicking the role of the Holy Spirit by pointing people to the false Messiah.

The second reason the number 666 is used is because of its symbolism in Jewish thinking. The Bible tells us that in the year in which King Solomon received 666 talents of gold, he turned his back on God and became obsessed with women, horses, and money (1 Kings 10:14 and 2 Chronicles 9:13). Thus, in Jewish history, the number 666 came to stand for apostasy.

The third reason the number is used is related to the fact that in both Hebrew and Greek, the letters of the alphabet also stand for numbers. This makes it possible to add up a numerical value for every name. The Antichrist's name will add up to 666 in either Greek or Hebrew (or perhaps both) and will thus be an expression of the ultimate apostasy - presenting one's self as a substitute for the real Messiah.

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